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Soft matter and liquids

Soft matter and liquids

The Mpemba effect

29 Mar 2006 Matin Durrani

Seemingly simple questions can lead to unexpected answers

Which takes less time to freeze: hot or cold water? The obvious answer from thermodynamics would be that cold water freezes first. But like other straightforward questions in physics, this one is far more complex than first meets the eye (see “Does hot water freeze first” Physics World April 2006 p19). Hot water, it seems, can freeze first, although not always. It all depends on what you mean by “freezing”, how hot the liquids initially are, how much gas is dissolved in the water, what shape the containers are and so on. While some may view these as tedious complications to a trivial problem, it is one that has been mused over by no less than Aristotle, Francis Bacon and René Descartes, and is now known as the “Mpemba effect” after a Tanzanian schoolboy. Noticing that an ice-cream mixture cooled faster when initially heated, Mpemba failed to get a satisfactory explanation from his teachers, and a full understanding of the effect still eludes us. The story illustrates that while it is right to be sceptical of unusual results, we should neither mock the simple question nor dismiss the unexpected answer out of hand.

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